The Artscroll commentary to the first pasuk of Vayeira says that Abraham pressed Ishmael into service to teach him about hospitality. From whence this comment about Ishmael?
They're coming from Genesis 18:7:
וְאֶל־הַבָּקָ֖ר רָ֣ץ אַבְרָהָ֑ם וַיִּקַּ֨ח בֶּן־בָּקָ֜ר רַ֤ךְ וָטוֹב֙ וַיִּתֵּ֣ן אֶל־הַנַּ֔עַר וַיְמַהֵ֖ר לַעֲשׂ֥וֹת אֹתֽוֹ
Then Abraham ran to the herd, took a calf, tender and choice, and gave it to a servant-boy, who hastened to prepare it.
Rashi there comments:
אל הנער. זֶה יִשְׁמָעֵאל, לְחַנְּכוֹ בְּמִצְוֹת (בראשית רבה):
TO THE YOUNG MAN — This was Ishmael whom he bade to do this in order to train him to the performance of religious duties (in this case the duty of hospitality) (Genesis Rabbah 48:13).
His comment comes from Bereishis Rabbah 48:13:
וַיִּתֵּן אֶל הַנַּעַר, זֶה יִשְׁמָעֵאל, בִּשְׁבִיל לְזַרְזוֹ בְּמִצְווֹת.
Eitz Yosef ad. loc. says this is evident from the fact that Avraham gave away the mitzvah. He wouldn't give it to anyone else, were it not for the mitzvah of chinuch. Also, Yishmael elsewhere is called נער (Genesis 21:12).